The present invention relates to a security document having a security thread embedded in the interior of the document, said thread being visually detectable in incident and transmitted light and having specific physical properties, and to methods for producing and testing the security document and the security thread.
It is known to prevent forgery by providing security documents such as money-value paper, identification cards, etc., with so-called "security threads". When a security document is being produced, these security threads are introduced into the interior of the sheet of paper during the sheet-forming process. The thread is usually embedded in such a way as to be completely surrounded by paper fibers (British Patent No. 8242 (1839), German utility model 72 18 681).
It is also known to prepare the security document or embed the thread in such a way that the thread is partially exposed and is thus directly visible or accessible at these places (German offenlegungsschrift No. 27 43 019). The specific optical properties of a security thread can be tested particularly well and easily when embedded in this manner.
German offenlegungsschrift No. 21 56 888 also discloses, among other things, the introduction of networks or individual threads made of thermoplastic material into security documents. Thermoplastic materials are used whose melting point is lower than 160.degree. C. Since these temperatures are reached during the production of the paper, e.g. during drying, these thermoplasts melt, thereby reducing the networks into individual islands of thermoplastic material which are firmly connected with the paper substance.
In order to ascertain the authenticity of a security document provided with a security thread, the paper is usually tested as to the presence of the security thread and its position in the paper by regarding it in incident and transmitted light. Security threads which are glued on or painted on can be distinguished in this way very easily from genuine security threads introduced during the production of the paper.
In the case of forged security documents in which the security thread is glued in between two finished layers of paper, the visual impression may in fact correspond to that of a security document having a security thread introduced in situ. However, such a forgery can be easily distinguished from genuine bank notes due to the different properties of the paper (stiffness, resistance to separation, etc.) and the resulting difference in the "feel".
It is known to allow for security threads to be machine tested as well by using threads having specific physical properties on the basis of which the threads can be identified in appropriate testing apparatus. Such security threads are, for example, threads having specific magnetic, electric or fluorescent properties (German Patent No. 16 96 245, U.S. Pat. No. 2,143,406).
The testing of the known security documents having security threads is therefore usually directed to visual and/or machine testing in which it is ascertained whether a security thread is embedded at all and, possibly in a second step, whether this thread has the required physical properties.
Since security threads are by definition supposed to be localizable without using any devices and may be removed from the security document relatively easily, possibly destroying the latter, they are exposed to analysis attacks to a special degree. In order to counter forgery to a greater degree the security thread should therefore not only have a special physical property and be elaborately embedded in the paper but also, if possible, be of elaborate production itself and be difficult for a forger to recreate. But this requirement should not in any way impede the economical industrial production of such security threads.